Generally, pulp is produced either chemically by digesting, mechanically by grinding or by refining or repulping recycled paper, so-called secondary fiber pulping, and contains a wide variety of impurities such as bark, pitch particles, grit, metallic substances, plastics and the like. Irrespective of the method utilized for the production of pulp, there is almost invariably a need of removing or separating the many impurities from the pulp before it is used in the production of paper.
Different methods used to eliminate these impurities from the pulp include knotters, centrifugal cleaners or hydrocyclones, rotary centrifugal screens and enclosed pressure screens. Pulp screened in pressure screens located in the screening department of the pulp plant is usually transferred into the screening department by a pump. For example, in known plants chemical pulp is first bleached by oxygen in a reactor from where the pulp is conducted into a blow tank for permitting excess oxygen to expand and to vent therefrom. Thereafter, the bleached pulp is pumped into a pressure screen by a pump.
However, it was surprisingly found that when oxygen was added in the bleaching operation preceding the prior art screen, the capacity of the screen decreased by about 20%. It has now been surprisingly found that this drop in capacity is to a great extent due to the increased gas content of the pulp based on the oxygen bleaching operation and to the fact that in the blow tank the gases will be vented only partially because of the high consistency of the pulp. Similar problems have been encountered in pulp washing and thickening operations and it has now been found, surprisingly, that these problems can be largely obviated by removing the gas from the fiber suspension prior to the respective treatment step.
As used throughout this specification (including claims), the word "gas" is intended to include any and all gases, whether free, combined or dissolved, including by way of example only air; and the expression "deaerating pump" or "degassing pump" is intended to mean a centrifugal pump capable of separating gas (as above defined) from the working liquid passing through the pump, which includes a gas channel for conveying separated gas from a zone upstream of or in front of the impeller to a zone downstream of or in back of the impeller, said pump further including a vent to permit the removal of said separated gas from the pump. Examples of suitable deaerating pumps are a pump sold as a degassing MC.RTM. pump by the assignee of the present invention, A. Ahlstrom Corporation, another pump sold by said assignee under the trademark AHLSTAR.RTM. equipped with AIRSEP.RTM. degassing. Also as used herein, the term "liquid" is intended not only to embrace liquids as conventionally defined but also slurries and suspensions which flow like liquids or are caused to flow through a deaerating pump like a liquid.
Gas is present in pulp suspensions mainly in three forms, namely, in the form of small bubbles, dissolved or chemical bound gas.
The chemically bound gas or dissolved gas seldom causes problems in the pulp and papermaking processes but can cause problems if conditions are changed and bubbles start to form.
Gas bubbles in the fiber suspension can be present as free bubbles in the liquid between the fibers or as bound bubbles attached to fibers. Both bound and free bubbles cause problems in the papermaking processes. Free bubbles cause special problems in the pulp and papermaking processes when they are present in too great an amount. The problems include foam problems, instability of the processes, decreased deaerating, and the like.
The method of the present invention relate to the separation and removal of most of the free air bubbles so that the problems caused by an excess amount of free air bubbles is eliminated.
Total gas removal is generally accomplished by another type of gas removal, so-called mechanical gas separation. With this method, all of the free and bound bubbles are removed. Also part of the dissolved gas is removed. This type of gas removal is performed immediately in front of the paper machine forming section to avoid pinholes and other problems on the forming wire. This method, which is described by K. D. Kurz, Tappi Engineering Conference, Sep. 19-21, (1978), is expensive and creates large amounts of foam when the fiber suspension is ejected with high speed onto a metal surface in a vacuum tank.
The traditional degassing assemblies in the pulp and paper industry are remarkably space demanding and hence costly, and the separated gas occurs in large volumes, from which reclaiming and conveying thereof is difficult. The most usual degassing equipment is a tank having a large diameter in which the gas in a gas contained liquid is permitted to rise to the liquid surface of the tank for removal. In order to be certain that a sufficient time period for degassing is given, the diameter of such a tank in large pulp plants can be 10-20 m and the height 5-6 m. It will thus be apparent to persons of ordinary skill that investment costs for a degassing tank of this kind are high and the reclaiming of gas therefrom is difficult.
Accordingly, it is an object of the present invention to eliminate the above-mentioned problems caused by the presence of air and/or gas in the pulp and to improve the efficiency of various pulp treatment processes especially as they relate to an increase in capacity of a screening department, a washing plant or a thickening department.
It is a further object of the present invention to remove air from pulp suspensions without simultaneously generating foam and problems associated with the creation of foam as they are known to occur, for example, in prior art deculators which have been unsuccessfully tested for use prior to washers. Eventually the use of deculators prior to washers had to be discontinued due to increasing foam problems.